The invention relates to a seal for a shut-off member for translatory movement which is moved from a closed position to an open position or in the opposite direction, in order to shut off flows of a substance.
The seal comprises metal components which define a pressure chamber to which external pressure can be applied in order to prevent leakage. The exclusive use of metal components permits use for wide temperature ranges and extraordinarily wearing conditions caused by frequency of operation of the valve or the composition of the working medium.
For the operation of such a seal it is important that the leakage resistance of the fitting be achieved between the casing and the shut-off member by means of two sealing seats disposed in the casing. When there is pressure difference in a particular direction, a fixed sealing seat on the side facing away from the pressure difference is necessary. The side facing the pressure difference must be given an axially movable seal seat that is able to equalize even deformations of the casing by internal pressure and/or external stress. Mechanical point stresses from positions outside of the casing necessitate a rigidity of the movable seat ring directly proportional to the distance between these points of application of force, the seat ring being no longer able to compensate for deformations of the shut-off member and casing. Thus, the overall system is not assured against leakage in every state of operation. To achieve the basic requirement of allowing no components of the medium to get into the housing whether the fitting is in the closed or open position, a pressure can be applied within the casing by means of a blocking medium. The result is that, in the event of leakage, only components of the blocking medium can enter the working medium and the shut-off for manual work downstream is secure.
Basically, sealing systems are known which utilize expandable sealing elements in order to produce sufficient surface pressure on the shut-off member. Tubular sealing elements of resilient synthetic resin materials are usable only to a limited extent on account of temperature limits and poor resistance to wear. Particularly in the case of advanced wear, such sealing elements tend to stick in the gap they are sealing. Therefore either the expandability of the sealing element or the possibility of operating the valve is lost.
Known metal sealing systems for providing axial flexibility have the disadvantage that hybrid solutions are involved, of a combination of metal surrounding the pressure chamber that is to be expanded externally and the transmission of the force thereby produced to resilient sealing elements made of elastomers. The disadvantages described above are again encountered. The pressure chambers are defined, for reasons of elastic deformability, by a plurality of parallel membranes whose seal against one another cannot be tested either during manufacture or in operation. The multilayer character of the elastic membrane is necessary because it must have the ability to deform even at the maximum working temperature. As for the function of sealing the pressure chamber, the failure of a single membrane leads to a general failure. It is extremely difficult to use forming processes to manufacture a pack of membranes to accommodate the elastic sealing ring, especially to make them of approximately equal radial thickness. The retention of the elastic sealing ring in the membrane pack is inadequate, so that wear of the elastic sealing ring differs according to its circumferential position in relation to the direction of the translatory movement of the shut-off member.